1. Field of the Invention
The present invention relates to a friction force microscope for measuring a friction force based on degree of a torsion of a cantilever in a scanning probe microscope.
2. Description of the Art
A friction force microscope is developed as an apparatus for measuring a friction force between a sample and a probe tip of a cantilever in a scanning probe microscope, and a basic principle thereof is as follows. Under a state in which the probe tip of the cantilever is brought into contact with the sample, the sample is scanned in a direction perpendicular to a longitudinal direction of the cantilever. Then, a displacement amount, at that time, in a torsional direction of the cantilever is obtained in each position, to thereby generate an image of a distribution of the friction force in a measurement range (see Japanese Patent Application Laid-open No. Hei 6-241762).
However, a conventional friction force microscope can easily measure a relative distribution of the friction force between the probe and the sample surface in the measurement range, but it is difficult for the conventional friction force microscope to measure an absolute value of the friction force. This is because the cantilever has a variation of characteristics. Therefore, it is difficult to use the apparatus in an application where friction forces of a plurality of samples are compared with each other by using a plurality of cantilevers.
Here, there is a problem as a variation of characteristics of the cantilever, which are parameters necessary for calculating the friction force from the measured torsional displacement signal, including (i) a torsional spring constant of the cantilever, (ii) sensitivity of a torsion signal of the cantilever, and (iii) a height of the cantilever probe.
It has been tried to measure those parameters for each cantilever so as to measure the absolute value of the friction force, and the following methods have been used: (i) using a torsional spring constant determined from dimensions of the cantilever; (ii) using sensitivity of the signal in a bending direction, which is regarded to be the same as sensitivity of the signal in the bending direction of the cantilever or to have a constant ratio to the sensitivity; and (iii) measuring a height of the probe by an electron microscope, an optical microscope, or the like.
However, the methods described above for correcting parameters of the cantilever have the following problems.
(i) It is difficult to accurately measure a thickness as a dimension of the cantilever, and because a spring constant in the torsional direction is proportional to the cube of the thickness, an original measuring error is enlarged. As a result, it is difficult to calculate an accurate torsional spring constant value.
(ii) Because of a spot shape and an intensity distribution of a laser beam applied to the cantilever, and unevenness of a reflective surface of the cantilever, sensitivity in the bending direction is not the same as sensitivity in the torsional direction, or does not have a constant ratio to the sensitivity in the torsional direction.
(iii) The probe tip of the cantilever is too small to observe by an optical microscope so that measurement is difficult. In addition, in the friction force microscope for measuring a friction force resulting from torsion of the cantilever in the scanning probe microscope, the height of the probe necessary for calculating the friction force is not a length from a surface of the cantilever to which the probe is attached to the probe tip but is a length from a rotation center of cantilever torsion to the probe tip. However, the rotation center of the cantilever torsion cannot be identified only by observing externally using a microscope or the like, and there is no means for determining from which part the length to the probe tip should be measured.